1. Field of the Invention
The invention concerns a magnetic resonance system of the type having an examination tunnel and a whole-body antenna with two connection terminals that extends like a cylinder around the examination tunnel along a longitudinal axis, and a radio-frequency supply device in order to respectively supply the whole-body antenna with radio-frequency signals for emission of an radio-frequency field in the examination tunnel. The radio-frequency supply device has a radio-frequency generator for generation of a radio-frequency signal, a signal splitter that splits a radio-frequency signal (arriving from the radio-frequency generator) to be emitted into two partial signals that are phase-shifted by 90° relative to one another, and two radio-frequency feed lines connected with the two connection points of the whole-body antenna, via which radio-frequency feed lines the two partial signals are fed into the whole-body antenna. Moreover, the invention concerns an antenna system for such a magnetic resonance examination system, a method for designing such a magnetic resonance system and a method to generate magnetic resonance exposures with a corresponding magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance tomography is a technique for acquisition of images of the inside of the body of a living examination subject that has become widespread. In order to acquire an image with this method, the body or a body part of the patient or test subject that is to be examined must initially be exposed to an optimally homogeneous static basic magnetic field which is generated by a basic field magnet of the magnetic resonance system. Rapidly switched gradient fields that are generated by what are known as gradient coils are superimposed on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, radio-frequency pulses of a defined field strength (known as the “B1 field”) are radiated into the examination subject with radio-frequency antennas. The nuclear spins of the atoms in the examination subject are excited by means of these radio-frequency pulses such that they are deflected from their equilibrium position by what is known as an “excitation flip angle” parallel to the basic magnetic field. The nuclear spins then process around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. The magnetic resonance images of the examination subject are ultimately created on the basis of the acquired magnetic resonance signals.
To emit the necessary radio-frequency pulses in the examination tunnel, the tomograph typically possesses a radio-frequency antenna permanently installed in the housing, which is also designated as a “whole-body antenna” or “body coil”. Typical designs for whole-body antennas are known as “cage structures” or “birdcage structures”, TEM or band antennas as well as saddle coils. Such a whole-body antenna is schematically shown with a cage structure in FIG. 1. The whole-body antenna 2′ has antenna rods 7′ parallel to the longitudinal axis L and arranged at various circumferential positions around the examination tunnel T, which antenna rods 7′ are respectively connected by annular antenna elements 5, 6 at the facing sides. The examination tunnel is thereby defined by the space enclosed by the antenna. Two radio-frequency feed lines 28, 29 of a radio-frequency supply device 20 are connected two connection points 3, 4 that are arranged offset from one another by 90° along the circumference on one of the annular antenna elements. FIG. 1 shows the design of such a radio-frequency supply device that is most frequently used at present. A radio-frequency signal RF is emitted by a radio-frequency generator 21 which possesses a suitable radio-frequency amplifier at the output side. This radio-frequency signal RF is fed into a first input 24 of a signal splitter 23. Said signal splitter 23 is hereby what is known as a hybrid module that divides the radio-frequency signal RF into two partial signals RF1, RF2 which are phase-shifted by 90° but are otherwise identical. These two partial signals RF1, RF2 are present at the outputs 26, 27 of the signal splitter 23 and there are provided on the radio-frequency feed lines 28, 29 to the connection points 3, 4 of the whole-body antenna 2′. A fourth input 25 of the signal splitter 23 is terminated with a 50Ω resistor 22 in order to accommodate powers reflected by the whole-body antenna 2′. In this design, there is a fixed weighting of the signals at the inputs of the antenna, and the antenna is designed such that it emits a circularly polarized radio-frequency field, meaning that the antenna can emit in precisely one circularly polarized mode MCP (which is symbolically represented by the circular line in FIG. 1).
In particular in newer magnetic resonance systems with basic magnetic field strengths greater than three Tesla, considerable eddy currents are frequently induced in the patient upon radiation of the radio-frequency pulses. As a result of this the actual homogeneous, radiated B1 field is more or less strongly distorted in the examination volume.
There are presently the following approaches in hardware in order to alleviate the problems caused by the field distortions:
Local field corrections can be achieved by the use of dielectric cushions or similar elements influencing the RF field that are placed on the patient.
A second possibility is shown in FIG. 2. This is a design similar to the design in FIG. 1, but respective amplitude and/or phase regulators 30, 31 with which the amplitudes and/or phases of the partial signals RF1, RF2 can be arbitrarily varied are additionally interposed after the outputs 26, 27 of the signal splitter 23 in the radio-frequency feed lines 28, 29. Correspondingly modified partial signals RF1′, RF2′ are fed into the whole-body antenna 2′ designed in the typical manner at the connection points 3, 4. Various radio-frequency modes can be excited in the whole-body antenna 2′ via a suitable phase and amplitude shift of the two partial signals RF1′, RF2′. Only two of these various radio-frequency modes, namely the circularly polarized mode MCP and an elliptically polarized mode MEP, are symbolically represented in FIG. 2. In the elliptically polarized mode MEP, a radio-frequency field is emitted which is elliptically polarized in a plane lying perpendicular to the longitudinal axis L and not circularly polarized (like the typical circularly polarized field). Which mode MCP, MEP is emitted depends on which amplitude ratio and which phase shift the two partial signals RF1′, RF2′ have relative to one another. If the amplitudes are equal and the phase shift is 90°, a circularly polarized field is emitted as in the exemplary embodiment according to FIG. 1. A disadvantage of this system is that additional components 30, 31 are required in order to affect the field in the desired manner. These components 30, 31 must be highly precise. A corresponding control technology is additionally required in order to be able to control the components 30, 31 as precisely as possible.
Individual settings of the amplitude values and phase values of a number of radio-frequency pulses emitted by various separate transmission channels are presently discussed as an additional promising approach to the homogenization of the B1 field. An example of this is explained in DE 101 24 465 A1. Such designs are naturally also relatively complicated and expensive.